1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft maneuvering.
2. Description of the Related Art
Spacecraft are directed through various maneuvers to guide them along a variety of celestial paths (e.g., interplanetary voyages and orbits around astronomical bodies such as the Earth) to perform a variety of functions (e.g., weather or planet surface monitoring, commercial communications and scientific observations). These spacecraft maneuvers include ones which initially place a spacecraft in an predetermined celestial path, ones which maintain a desired spacecraft station in the celestial path and ones which maintain a desired spacecraft attitude in the celestial path. In an exemplary case in which the celestial path is a geosynchronous Earth orbit, these spacecraft maneuvers are typically referred to as orbital transferring, stationkeeping and attitude controlling.
Spacecraft maneuvers are accomplished by the application of forces and torques to the spacecraft. Typically, orbital transferring and stationkeeeping are achieved by directing force vectors through the center of mass of spacecraft so as to obtain changes in spacecraft position without disturbing spacecraft attitude. In contrast, attitude controlling is typically achieved by spacing force vectors from the center of mass to thereby generate torque vectors which realize attitude changes.
Conventional spacecraft structures for application of spacecraft forces and torques include thrusters, momentum and reaction wheels, surfaces which receive solar pressure (e.g., solar cell arrays), extended masses which interact with ambient gravity gradients, mechanical inter-spacecraft control structures (e.g., mechanical arms) and magnetic torquing coils which interact with ambient magnetic fields (e.g., the Earth's magnetic field).
Although all of these control structures have been used to effect spacecraft maneuvers, each has characteristics that limit their usefulness. Thruster systems are typically bulky, heavy and expel propellant products which can coat and degrade sensitive spacecraft structures (e.g., electro-optical instruments and solar cell arrays). In addition, fuel is an expendable substance of limited supply and, hence, its lack routinely produces the effective end of useful spacecraft life. Momentum and reaction wheels are restricted to the application of torques and their momentum must be periodically "dumped" with other control structures (e.g., limited-fuel thrusters) when it approaches the design limit of the wheels.
Generation of spacecraft forces and spacecraft torques by use of solar pressure and ambient gravity gradients typically requires the arrangement or deployment of mechanical structures (e.g., selective rotation of solar cell arrays or extension of gravitational masses from the spacecraft on booms or tethers). Alternatively, generation of spacecraft forces and spacecraft torques can be effected with the limited quantity of thruster fuel.
Inter-spacecraft mechanical control structures are typically bulky and heavy which means they use a significant amount of spacecraft volume and weight (quantities which are always in short supply). Such structures can only be applied when the spacecraft spacing is less than the maximum reach of the control structures. In addition, direct-contact mechanical control may initiate a damaging electrostatic discharge because of spacecraft potential differences, may cause mechanical damage and may respond to electrical or mechanical failure by failing to uncouple the spacecrafts.
The application of magnetic fields to spacecraft maneuvers has typically been directed to the use of ambient magnetic fields (e.g., the Earth's magnetic field) or to the theoretical use of magnetic structures that have been previously distributed in orbit about a celestial body (e.g., see Lebon, Benoit A., "Magnetic Propulsion along an Orbiting Grain Stream", Journal of Spacecraft and Rockets, Vol. 23, March-April, 1986, pp. 141-143).
The forces and torques generated in magnetic structures by an ambient magnetic field have been generally described by many investigators (e.g., see Boyer, Timothy H., "The Force on a Magnetic Dipole", American Journal of Physics, August 1988, Vol. 56, No. 8, pp. 688-692; Brownstein, K. R., "Force Exerted on a Magnetic Dipole", American Journal of Physics, October 1993, Vol. 61, No. 10, pp. 940-941); Greene, Jack B., et al., "Force on a Magnetic Dipole", American Journal of Physics, February 1971, Vol. 39, pp. 172-175; Hnizdo,V., "Hidden Momentum and the Force on a Magnetic Dipole", Magnetic and Electrical Separation, 1992, Vol. 3, pp. 259-265; and Vaidman, Lev, "Torque and Force on a Magnetic Dipole", American Journal of Physics, October 1990, Vol. 58, No. 10, pp. 978-983).
Magnetic forces on neutrally charged objects are not induced by uniform magnetic fields. Accordingly, ambient magnetic fields cannot be used to generate forces on spacecraft because they are essentially uniform at the spatial scale of spacecraft. In addition, the generation of torques with an ambient magnetic field is limited in application because the direction of the ambient magnetic field gradient cannot be selected.